Orthogonal Frequency Division Multiplexing (OFDM) is one of the Multi-Carrier Modulation (MCM) technologies. OFDM divides a frequency channel or frequency carrier into multiple orthogonal sub-channels each associated with a frequency subcarrier and allows for transmission of sub-data streams in parallel at low rates as opposed to a single data stream over a single frequency carrier at a much higher rate. The low-rate sub-data streams each modulate one of the sub-carriers. At the receiving end, the received orthogonal subcarriers each carrying a sub-data stream can be separated from one another using appropriate techniques to reduce inter-channel interference (ICI) between the sub-channels. Because the bandwidth of each sub-channel is smaller than the bandwidth of the channel, fading in each sub-channel generally can be considered flat across the frequency bandwidth of that sub-channel. As a result, inter symbol interference may be eliminated. In addition, because the bandwidth of each sub-channel is only a small portion of the bandwidth of the original channel, channel equalization also becomes easier.
The Long Term Evolution (LTE) project is directed to developing wireless communications standards as an evolution from 3G technology. The project began at the 2004 3rd Generation Partnership Project (3GPP) Conference held in Toronto. LTE uses OFDM and MIMO (Multiple-Input Multiple-Output) technologies. With a 20 MHz spectral bandwidth, LTE can provide a downlink peak rate of 326 Mbit/s and an uplink peak rate of 86 Mbit/s. LTE can improve user experience at cell edges, increase cell capacities, and reduce system latencies.
An LTE network may include a number of cells, each corresponding to a geographical area. Within each cell, mobile devices such as mobile phones access network services such as phone services or Internet services through an interface station such as a base station, which is also referred to as eNode B in LTE terminology.
A mobile device often needs to measure the signal-to-noise ratio (SNR) or the noise in a received signal for the purposes of performing Minimum Mean Square Error (MMSE) equalization, computing Channel Quality Indicator (CQI), and so on. Because some neighbor cells in an LTE cellular network operate at the same center frequency, the noise power of a received signal can vary widely due to interference from the same-frequency neighbor cells. For such received signals, conventional methods for estimating the noise in an entire band cannot be used to measure the noise or to estimate the signal-to-noise ratio.
Conventional methods for estimating noise or signal-to-noise ratio in subcarriers or sub-channels include constellation diagram-based and idle subcarrier-based methods. For example, through a constellation diagram, noise or signal-to-noise ratio can be estimated based on expected values and variances derived from equalized data. Statistical error, however, may be significant when a high-order modulation is used on a physical channel or when the signal-to-noise ratio is relatively low. Therefore, estimating noise or signal-to-noise ratio using a constellation diagram does not perform well with a large dynamic range of signal-to-noise ratio.
Alternatively, an idle subcarrier carrying no signal can be used to estimate the noise power. Channel estimate of a subcarrier carrying signals provides the signal power. Thus, the signal-to-noise ratio can be calculated by comparing the noise power and the signal power. Noise estimation based on an idle subcarrier, however, assumes uniform noise level across sub-bands and does not provide for accurate calculation of signal-to-noise ratio for a sub-band consisting of multiple subcarriers, when noise varies widely within a sub-band.
International Patent Application No. WO2011088501 (A1) provides another example of a conventional method for noise estimation and calculation of signal-to-noise ratio.
There is a need for a noise estimation method that performs well over a large dynamic range of signal-to-noise ratio, and/or provides an accurate measurement of signal-to-noise ratio for each sub-channel.